Significant amount of attention has been placed on developing new technologies for providing energy from resources other than fossil fuels. Biomass is a resource that shows promise as a fossil fuel alternative. As opposed to fossil fuel, biomass is also renewable.
One type of biomass is plant biomass. Plant biomass is the most abundant source of carbohydrate in the world due to the lignocellulosic materials composing the cell walls in higher plants. Plant cell walls are divided into two sections, primary cell walls and secondary cell walls. The primary cell wall provides structure for expanding cells and is composed of three major polysaccharides (cellulose, pectin, and hemicellulose) and one group of glycoproteins. The secondary cell wall, which is produced after the cell has finished growing, also contains polysaccharides and is strengthened through polymeric lignin covalently cross-linked to hemicellulose. Hemicellulose and pectin are typically found in abundance, but cellulose is the predominant polysaccharide and the most abundant source of carbohydrates.
Most transportation vehicles, whether boats, trains, planes and automobiles, require high power density provided by internal combustion and/or propulsion engines. These engines require clean burning fuels which are generally in liquid form or, to a lesser extent, compressed gases. Liquid fuels are more portable due to their high energy density and their ability to be pumped, which makes handling easier. This is why most fuels are liquids.
Currently, biomass provides the only renewable alternative for liquid transportation fuel. Unlike nuclear and wind applications, and for the most part solar resources, biomass is capable of being converted into a liquid form. Unfortunately, the progress in developing new technologies for producing liquid biofuels has been slow in developing, especially for liquid fuel products that fit within the current infrastructure. Although a variety of fuels can be produced from biomass resources, such as ethanol, methanol, biodiesel, Fischer-Tropsch diesel, and gaseous fuels, such as hydrogen and methane, these fuels require either new distribution technologies and/or combustion technologies appropriate for their characteristics. The production of these fuels also tend to be expensive and raise questions with respect to their net carbon savings.
Ethanol, for example, is made by converting the carbohydrate from biomass into sugar, which is then converted into ethanol in a fermentation process similar to brewing beer. Ethanol is the most widely used biofuel today with current capacity of 4.3 billion gallons per year based on starch crops, such as corn. Ethanol, however, has very substantial disadvantages with respect its energy value as a fuel relative to the amount of energy needed to produce it. Ethanol produced by fermentation contains large amounts of water, typically comprising only about 5 percent of ethanol by volume in the water/alcohol fermentation product. The removal of this water is highly energy-consuming, and often requires the use of natural gas as a heat source. Ethanol also has less energy content than gasoline, which means that it takes more fuel to go the same distance. Ethanol is very corrosive to fuel systems and cannot be transported in petroleum pipelines. As a result, ethanol is transported over-the-road in tank trucks, which increases its overall cost and energy consumption. When considering the total energy consumed by farm equipment, cultivation, planting, fertilizers, pesticides, herbicides, petroleum-based fungicides, irrigation systems, harvesting, transportation to processing plants, fermentation, distillation, drying, transport to fuel terminals and retail pumps, and lower ethanol fuel energy content, the net energy content value added and delivered to consumers is very small.
Biodiesel is another potential energy source. Biodiesel can be made from vegetable oil, animal fats, waste vegetable oils, microalgae oils or recycled restaurant greases, and is produced through a process in which organically derived oils are combined with alcohol (ethanol or methanol) in the presence of a catalyst to form ethyl or methyl ester. The biomass-derived ethyl or methyl esters can then be blended with conventional diesel fuel or used as a neat fuel (100% biodiesel). Biodiesel is also expensive to manufacture, and poses various issues in its use and combustion. For example, biodiesel is not suitable for use in lower temperatures and requires special handling to avoid gelling in cold temperatures. Biodiesel also tends to provide higher nitrogen oxide emissions, and cannot be transported in petroleum pipelines.
Biomass can also be gasified to produce a synthesis gas composed primarily of hydrogen and carbon monoxide, also called syngas or biosyngas. Syngas produced today is used directly to generate heat and power, but several types of biofuels may be derived from syngas. Hydrogen can be recovered from syngas, or it can be catalytically converted to methanol. The gas can also be run through a biological reactor to produce ethanol or converted using Fischer-Tropsch catalyst into a liquid stream with properties similar to diesel fuel, called Fischer-Tropsch diesel. These processes are expensive and generate fuels that are not easily assimilated in current transportation technology. Processes capable of converting biomass using catalytic techniques would be especially advantageous due to its familiarity within the current fuel industry.